Apparatus for measuring selected engine vibration characteristics



July 23, 19468 w; F. BROWN ET AL 3,393,557

APPARATUS FOR MEASURING SELECTED ENGINE VIBRATION CHARACTERISTICS FiledJune 29, 1967 3 Sheets-Sheet 1 July 23, 1968 w. F. BROWN ET AL APPARATUSFOR MEASURING SELECTED 3,393,557 ENGINE VIBRATION CHARACTERISTICS v 5Sheets-Sheet 2 Filed June 29, 1967 -NN t.: N .p1

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APPARATUS FOR MEASURING SELECTED ENGINE t VIBRATION CHARACTERISTICS vFiled June 29, 1967 3 Sheets-Sheet :5

United States Patent O 3,393,557 APPARATUS FOR MEASURING SELECTED ENGINEVIBRATION CHARACTERISTICS William F. Brown, Wappingers Falls, and RonaldJ. Goetchius, Fishkill, N.Y., assignors to Texaco Inc., New York, N.Y.,a corporation of Delaware Continuation-impart of application Ser. No.517,252, Dec. 29, 1965. This application June 29, 1967, Ser. No. 652,991

13 Claims. (Cl. 73-116) ABSTRACT OF THE DISCLOSURE An apparatus formeasuring selected engine 'vibration characteristics comprising atransducer for translating vibrations to electrical signals, having itsoutput connected to a pair of parallel channels each includingfrequencyselective and peak-reading means, one channel passing signalsonly of the vibration characteristic of interest and the other channelpassing signals of another predetermined frequency, each of thepeak-reading means producing a signal proportional to the peak-value ofthe electrical signal passed therethrough and coupled to a differencemeasuring means capable of producing a signal proportional to thedifference between the peak-values which is utilized in a meterindicative of the amount of the selected vibration characteristics.

This invention is a continuation-impart of application Ser. No. 517,252,`filed on Dec. 29, 1965, and no-w abandoned.

Background of the invention (l) Field of the invention- This inventionrelates to an apparatus for measuring a selected engine vibrationcharacteristic such as engine rumble or knock and more particularly toapparatus for detecting rumble or knock or both in gasoline enginesduring combustion.

(2) Description of the prior ara-Engine rumble is a low frequency noisecaused by crankshaft vibrations, such as bending or torsional Vibrationsproduced by uneven application of power to torque the crankshaft. Rumbleoccurs often in gasoline engines and is generally considered to be dueto improper combustion caused by carb-on deposits in theengine..Considerable difficulty has been encountered in measuring rumbledue toits transient nature, that is it occurs usually in short bursts.During engine speed changes such as in acceleration or deceleration, thelevel of background noise and vibration changes. This adds furtherdifficulty in obtaining a measure of rumble. Previously, rumble ingasoline engines had been measured at a predetermined engine speed bydetecting the noise intensity level increases in the rumble frequencyrange of the enginenoise frequency spectrum. Once these intensity levelincreases are measured, theyare assigned values with respect to areference noise level which is the normal sound intensity in thisfrequency range due to engi-ne noise when no rumble is present. Rumblemeasurements determined in this manner are adequate for use in enginesoperating at constant speed but have proved to be inadequate whereengine speed is varying rapidly as in acceleration. During acceleration,for example, the normal engine noise increases in intensity, in thefrequency range of interest such thatan erroneous indication of rumbleis obtained Iwhere actually no rumble is present.

Engine knock is an audio frequency noise characterized by an audibleping emanating from the engine cylinders. During a no-knock condition,the spark ignites the gases closest to the spark plug andthe fla-mefront continues in a Wave like manner to combust all the gas in thecylinder from the point of ignition in the cylinder 3,393,557 PatentedJuly 23, 1968 "ice chamber to the furthermost regions of the combustionchamber. During knock conditions the compressed gases in a cylinder ofan engine are no longer combusted by a smoothly progressive llame frontignited by a spark from a spark plug.

When the compressed gases at the end of the cylinder away from the sparkplug ignite almost instantaneously prior to the arrival of thecombusting flame front, ignited by a spark from the spark plug ignition,knock occurs. This knock condition arises as the unburnt portion of thecharge is progressively heated, due to the direct heat transfer from theflame and to the compression by the expanded burnt products. Knockconditions then may arise where these fresh gases reach a state ofcombustion before being ignited by the advancing llame. Under knockconditions, the end gases ignite almost instantaneously, and a violentpressure rise occurs followed by vigorous vibrations not only of thegases but of the material of the combustion chamber itself.

A moderate degree of knock can be tolerated in an automobile engine fora short period but, if extended, will eventually lead to overheating,loss of power, and mechanical damage. With airplane engines, knock couldnot be detected over the general noise level, and thus cannot betolerated under any circumstances. If any hot-spots are created in thecombustion chamber, as with overheated plugs or exhaust valves, thesemay themselves act as ignition forces so that auto ignition occurs,leading in turn to perignition. With the former phenomenon, the mixtureis ignited independently of the spark but at about the normal timing,and is only detectable by the engine running-on after switching off theelectric ignition. Pre-ignition represents a .more severe case whereignition occurs before its normal timing, and a very serious conditioncan quickly be reached.

Knock and preignition can develop from each other, a-nd can both lead tosevere damage and engine failure. Damage due to knock is usually notedas piston erosion, the violent pressure Iwaves physically removingparticles from the piston crown, whereas preignition failures areobserved as piston melting due to the excessive heat.

Since knock places a great deal of stress as Awell as wear and tear onthe piston and other moving parts of the engine much research has beencarried on in trying to obtain a simple apparatus capable of detectingthe noise intensity level in the knock frequency range of the enginenoise frequency spectrum. A further complication is that during engineoperation when an unduly heavy Iwork load is placed `on the engine suchas in accelerating up an incline, the level of background noise andvibration changes. This adds further diiculty in obtaining a measure ofknock since the masking background noise frequency and intensity arecontinually varying.

Apparatus has been heretofore used to measure knock intensity,particularly those recommended by the American Society for TestingMaterials. (See, for example, ASTM Manual for Rating Motor Fuels byMotor and Research Methods, Fourth edition, 1960; Published by theAmerican Society for Testing Materials, Philadelphia, Pa.; especially atpages 50 to 5l.) However, these prior apparatus are useful only formeasuring the octane rati-n-g of fuels. The apparatus described here isintended primarily for measurement of fuel octane requirement ofgasoline engines although it is adaptable as well to the measurement offuel octane.

Summary of the invention Accordingly, it is the main object of thepresent invention to provide a method and appar-atus for measuringselected engine vibration characteristics in the presence of varyingbackground noise and vibration.

It is another object of the present invention to pro- Ivide a method andapparatus for measuring engine knock or rumble in the presence ofvarying background noise and vibration.

It is a further object of the present invention to provide apparatuscapable of -measuring and displaying the peak intensity of knock orrumble emanating from a variable speed gasoline engine.

It is still another object of the present invention to provide apparatuscapable of measuring engine knock or rumble in which the gain and bandwidth of the apparatus can be easily adjusted for adapting the apparatusto engines having different characteristics.

According to the present invention, the apparatus for measuring aselected vibration characteristic of an engine such as rumble or knockcomprises a transducer means for converting engine vibrations intocorresponding electrical signal variations which are applied to a firstand second channel connected in parallel. Each channel includes afrequency-selective means and a peak-reading means. One of thefrequency-selective means passes the electrical signals falling withinthe predetermined selected vibration characteristic frequency rangewhile the other frequency-selective means passes the electrical signals4from a second predetermined engine vibration frequency range. Thepeak-reading means in each channel produces a signal proportional to thepeak-value of the electrical signals passed by the associatedfrequency-selective means. Subtracting means are provided for producinga difference signal proportional to the difference between thepeak-value signal derived from the peak-reading circuits of therespective channels. This difference signal is indicative of the amountof selected vibration characteristic produced by the engine.

The above-mentioned and other features and objectives of this inventionand thc manner of obtaining them will become more apparent and theinvention itself will be best understood by reference to the followingdescriptions of an embodiment of the invention taken in conjunction withthe accompanying drawings wherein:

Brief description of the drawings FIG. 1 is a schematic block diagr-amof the apparatus for measuring engine rumble;

FIG. 2 is a schematic circuit diagram of the band pass amplifier and thepeak reading circuit of the rumble or knock passing channel; and

FIG. 3 is a schematic block diagram of the apparatus for selectivelymeasuring either engine knock or engine rumble.

Description of the preferred embodiments Before mechanizing the instantinvention, it is necessary to determine the frequency range in which therumble or `knock noise occurs and other frequency characteristics of thetype engine of interest in order to determine the frequency-selectivecharacteristics of the bandpass amplifiers in the channels. This isaccomplished by a frequency analysis of the engine vibrations durin-grumbling and non-rumbling operations as well as knocking andnon-knocking operations, which consists of utilizing the accelerometeror other pick-up means to convert the engine vibrations intocorresponding electrical signals which may be recorded or otherwiseutilized so as to determine therefrom the frequency range in which therumble noise occurs and the frequency range in which the knock noiseoccurs and the frequency ranges which are unaffected by knock or rumble.The lgain of channels can be easily adjusted by measuring, with theapparatus of the instant invention, the noise characteristics of theengine when no rumble is present and then when no knock is present.Under such a condition each channel can be adjusted for gain such thatthe output signal from each channel is of the same amplitude.

The frequency analysis should be performed over the 60 to 1,600 cyclesper second (c.p.s.) range which has been determined to be the principalrange in which the frequency of the rumble phenomenon lie. It has beendetermined that there is a marked increase in noise amplitude in the 850to 1250 c.p.s. frequency range compared -to that observed `when norumblewasl detected. This later -frequency range will hereafter bereferred to as the rumble band. The engine noise band range for rumblemeasurement has been selected as the 400 to 700 c.p.s. frequency'bandwherestrong resonant modes due to engine vibration exist which areunaffected by rumble. This400 to 700 c.p.s. frequency' band willhereafter be referred to as the first engine noise band. However, thisfirst engine noise band can include other bandwidths which areunaffected by ythe `particular selected engine vibration characteristic.

A typical illustration of frequencies found on an analysis performedover the 600 to 8000 c.p.s. range on -a 1966`Ford V-S engine, whileanalyzing for a distant knock band indicated a marked increase in noiseamplitude in various areas of bandwith such as the 1600 to 2700 c.p.s.frequency range, and the 4600 to 5600 c.p.s. band range compared to thatobserved when no-knock was detected. Additionally, the 2700 to 3700c.p.s. area varied but slightly with knock as well as rumble intensity;in an embodiment of a typical meter this region can be used for thereference lmeasurement channel for both rumble and knock. lt is to beunderstood however, that any convenient bandwith c-an be used withinthis region as a reference channel.

The 1600 to 27 00 c.p.s. frequency range is close to the rumble range of850 to 1250 c.p.s., therefore, since the higher frequency range of 4600to 5600 c.p.s. is available this higher frequency range is preferred andWill be referred to as the .knock-frequency band. It is to be understoodthat `while this frequency range is a preferred range, other suitablefrequency ranges may be used as the knock frequency band.

The preferred engine noise band range for the knock frequency detectionhas been selected as the 2700 to 3700 c.p.s. frequency band which isunaffected by knock or rumble. This preferred 2700 to 3700 c.p.s.frequency band will hereafter be designated as the second engine noiseband but it will be understood that other bandwidths unaffected by knockwill be comprehended by this designation. i l

Referring to FIG. 1, the engine under observation is preferably agasoline engine. A transducer means such as an accelerometer 14 isattached as convenient to the engine block. For rumble measurements thisis preferably adjacent to the crankshaft rear center main bearingsupport. IFor knock measurements other placements are just asconvenient, e.g. adjacent spark plug openings. It will be appreciatedthat the same purpose can be achieved using a microphone adjacent theengine rather than an accelerometer. The accelerometer may be of thepiezoelectric type in which the bending of a crystal underaccelerationproduces an electrical charge or current which is linearly proportionalto the applied acceleration. The output of the accelerometer is theeletcrical equivalent to the rate of change of the engine blockvibration over a frequency range determined by the frequency responsecharacteristics of the accelerometer and its associated circuitry. Anexample of a suitable accelerometer is the Columbia ResearchLaboratories 504-2HT which has a nominal sensitivity of 50 mv./ g. andcapacitance of 25 picofarads including connecting cable capacitance anda fiat frequency response from l cycles per second to"10,000 c.p.s. whenoperated into a 6 megohm load which is pro-` vided by the preamplifier16. In addition, the preamplifier 16 acts as a low impedance source todrive the cable which connects the preamplifier. accelerometercombination at the engine tothe rest of the apparatus which is locatedremotely. To protect against electrical pickup caused by the engineignition system interfering with the rumble r or knock measuringapparatus, lthe cables connecting the accelerometer, the preamplifierand the rest of the equipment are runthrough metal conduit and thepreamplifier'itself is enclosed in a metal case; shielded coaxial cabledoes not provide adequate protection against this interference 'in thisapplication. The electrical signals upon arriving at the remote locationare amplied by a conventional band-pass amplifier 18 which is preset topass the frequencies from 300 to 1500 c.p.s. This amplifier lhas apass-band Voltage gain which is adjustable from 0 to 45 and employsresistance-capacitance filters in its input and output circuits tolachieve a equency response roll-off of six decibels per octave at thelow and high 'frequency cut-off points. The band limiting provided bythis amplifier 18 serves the purpose of eliminating the overlapofharmonic side-band frequencies generated 'in the amplitudenmodulator 22connected thereto. The resulting signal at the output of'the band-passamplifier 1'8 modulates a4 kc./s. 7square wave carrier signal generatedby a tuning fork oscillator'20. The net effect of the ,modulator is toshiftl the information in the input signal to frequency side bands atvthe odd harmonics of the 4 kc/s. carrier signal; the information carriedby the vibra- Ition signal is retained in any one of the side bands andmay be recovered through theV use of appropriate bandpass filters. Themodulation or the frequency transformation provided thereby permits theuse of less elaborate filters in the subsequent frequency selectiveamplifiers. Actually the modulator is a voltage control switch actuatedat a'4000 c.p.s. rate by the square wave oscillator 20 such that aproduct relatio'nship'is formed between the input vibration signalandthe switching signal. The resultant output fromthemodulator is asingle ended amplitude modulated 4000 c.p.s. squarewave.

As mentioned previously 'the rumble lies, generally in the frequencyband range of 850 to'V 1250 c'.`p.s; while the original first enginenoise band range" extends from 400 to. 700 c.p.s. In the transformer ormodulated signal, the frequencies corresponding to the rumble lie in the4850 to`5250 c.p.s. Vband while the,original engine noise band isshifted to the 4400 to 4700 c.p.s. band. As shown in FIGURE 1, theband-pass amplifiers 24 and26 in channels 1 and 2, respectivelycorrespond to these frequency bands in the preferred embodiment formeasuring rumble. Each of the band-pass amplifiers consists of twostagger-tuned frequency selective amplifiers which employ twin-Tfrequency rejection networks in a negative feed back configuration' togenerate the desired frequency response characteristics for eachchannel. In each of channels 1 and 2 of FIGURE 1, correspondingrespectively to the rumble band and the reference or engine noise band,there is a peak reading circuit, 28 and 30, respectively. These peakreading circuits each consist of a rectifier circuit at the output ofeach band-pass amplifier which producesl a DC signal proportional to theamplitude of the signal in each channel. In this way an analog signal isgenerated in each channel, one analog signal being characteristic of thecombined rumble and engine vibration intensities (channel 1) a secondbeing characteristic of the engine vibration intensity alone (channel2). These analog signals from the first and second channels are thenapplied to a conventional differential amplifier 32 which provides anoutputsignal proportional to the difference between the two analog inputsignals of channels 1 and 2. The lpeak value of this net differencesignal from differential amplifier 32 is detectedl and displayed by aconventional peak reading meter circuit 34, the display being held untilreset in order to facilitate recording of the resultant meter reading bythe operator. Hold and reset circuit-36 is provided for this purpose.This difference signal from the differential amplifier 32 represents therumble present in the engine since the engine noise which was present inboth channels (i.e. channel 1 and 2) was cancelled in the differentialamplifier leaving only that noise produced by the rumble. f

The particular differential amplifier utilized has a differentialvoltage gain of 10 and a common mode rejection of about which, inpractice, provided reasonably large signalsat the differential amplifieroutput when rumble or knock was present and nearly perfect cancellationofthe engine acceleration when neither rumble nor knock was present.

A further embodiment of this invention is depicted in block diagram inFIGURE 3 and includes beside channels 1 and 2 of the rumble meter,channel 3 which is a knock hand. In FIGURES 1 and 3, similar apparatushave been given the same reference numerals. A switch means 19 such as asingle pole, double throw switch is shown, by which the meter can beused to detect rumble, using channels 1 and 2, in a first switchposition and'knock, using channels 2 and 3, in a second switch position.The noise band, channel 2, is the same for both rumble and knockdetection.

As described above in connection with FIGURE 1, a frequency analysis wasperformed' for a typical example over the 600 to 8000 c.p.s. range on a1966'Ford V`8 engine. f

The preferred knock frequency band in this example is selected as 4600to 5600 c.p.s. and the preferred second engine noise band range, for theembodiment depicted in FIGURE 3, is selected as the 2700 to 3700 c.p.s.frequency band.

I When the switch means 19 of FIGURE 3 is switched to the rumbledetection position, the instrumentation is the same as that describedabove for FIGURE 1. However, when the switch means 19 is placed in theknock detection position, additional circuitry follows the preamplifier16. The band pass amplifier 124 in channel 3 is preset to passvfrequency band of 4600 to 5600 c.p.s., which has been referred to aboveas the preselected knock frequency band for this example. The output ofband pass amplifier 124, which consists of two stager tuned frequencyselective amplifiers similar to band pass amplifiers 24 and 26respectively in channels 1Y and 2 as described above, is passed to peakreading circuit 128. Peak reading circuit 128 is similar to peak readingcircuits 28 and 30 of channels 1 and 2 respectively. The analog signalfrom the second and third channels are then applied to a conventionaldifferential amplifier 132 Whichv provides an output signal proportionalto the difference between the two analog input signals of channels `2and 3. The output `of differential amplifier 132 in turn provides anoutput signal which is detected and displayed by the peak reading metercircuit 34 similar tothe meter circuit in the first embodimentillustrated in FIGURE 1 and described above. Differential amplifier 132is similar to differential amplifier 32 in channel 1. Hold and resetcircuits 136 are provided to hold the display on meter circuit 134 untilreset. The different signal from the differential amplifier 132represents the knock present in the engine noise which was present inboth channels (i.e. channels 2 and 3) is cancelled in the differentialamplifier 132 leaving only that noise produced by the knock.

Referring to FIGURE 2, there is shown an example of the circuitry-whichcan be utilized forvband-pass amplifier 24 and peak reading circuit 28of the rumble band, channel 1. The circuits in channels 2 and 3substantially correspond to those in channel 1 except for the values ofsome of the components utilized and, thus, only the details of the onechannel are shown. For rumble, when the frequency is below 2000 c.p.s.,the band-pass amplifier 24 receives the amplitude modulated waves frommodulator 22 over electrical connection 38 which contains an RCcou-pling network 40 which is connected to the control grid of pentodetube 42 of AC amplifier stage 44 (see FIGURES 1, 2 and 3). The amplifiedoutput of the pentode tube 42 of amplifier stage 44 is taken from theplate circuit over lead 46 containing capacitor 48 which is connected tothe control grid of triode tube 52 of amplifier stage 44. Triode 52.Yserves as a cathode follower with the output thereof connected overconducting'lead `54,-contair'iing capacitor 56 to RC coupling network 8of amplifier stage 60. Amplifier stage 60 is an exact replica ofamplifier stage 44. As can be seen these stages are connected in series.Each of amplifier stages 44 and 60 has what is popularly known as atwin- T frequency rejection network connected as a negative feedbackcircuit. The feedback circuit extends from output lead 54 of amplifierstage 44 via lead 57 which connects to one side of the associatedfrequency rejection network 62, the output of which is connected vialead 64 to the control grid of pentode tube 42 of amplifier stage 44.The frequency rejection network is arranged to reject,-that isattenuate, the desired frequencies and pass all others such that allother frequencies are cancelled bythe lnegative feedback except thosethat are rejected by the frequency rejection network in the feedkbackpath. The second amplifier stage 60 is exactly the same as amplifierstage 44 except for the components in the frequency rejection network 66in the feedback path which is arranged to reject different frequenciesthan those rejected by frequency rejection network 62. Accordingly, eachamplifier stage with its frequency dependent feedback network representsa singly tuned circuit with a response characteristic centered about afrequency determined by the twin-T components and a selectively (bandcenterv to band width ratio) determined by the amplifier gain withoutfeedback. Through appropriate stagger-tuning of the frequency selectiveamplifier pair in each channel, a channel frequency responsecharacteristic fiat to within one decibel in the pass-band with a highrate of response roll-off on either side of the pass-band was achieved.In the instant application, the required selectivity for each frequencyselective amplifier was approximately 2() and an insertion gain of aboutdb was obtained for each channel.

The peak reading circuit 28 is also shown in FIG. 2 which consists of asilicon diode 68 and a low pass filter connected to the output ofamplifier stage 60. The low pass filter consists of a parallel connected100K resistor 69 and a 1 microfarad condenser 70. This arrangementgenerates a DC signal proportional to the am-plitude of the signalpassed by the associated band-pass amplifier. The output lead 72 fromthe peak reading circuit is connected to the differential amplifier 32.

The output of the rumble noise channel 1 or the knock noise channel 3may be adjusted by means of the potentiorneter 74 shown before the peakreading circuit 28 at the output of amplifier stages `60 in FIG. 2. Thisadjustment is utilized to match the non-rumble or nonknock noiserespectively in the rumble or knock channel to the noise in the enginenoise channel so that these background and engine induced noises willexactly cancel in the differential amplifier. This matching is doneprior to making the rumble or knock measurement, at a time when there isno rumble or knock present. Effectively, the adjustment of thepotentiometer adjusts the gain of the channel.

It has been found that most automotive gasoline engines of the same typeexhibit similar vibration patterns because of the structuralsimilarities, especially with respect to the crankshaft which is theprincipal rumble noise source. However, it can be expected that due toslight difference in physical configuration, engines of different typesmay have vibration modes which differ slightly in frequency from thoseset forth above. As a result, this rumble or knock measurement systemmay be adjusted to operate with practically all engine types byreturning and recalibrating in accordance with the vibration frequencyanalysis previously described to select the appropriate rumble or knockand engine noise channels for each engine type involved.

We claim:

1. Apparatus for measuring a selected engine vibration 8 characteristiccomprising transducermeans for converting engine vibrations related tosaid characteristic into corresponding electrical signal variations, apair of channels connected in parallel to said transducer meansforreceiving said electrical signal variations, each channel including afrequency-selective means and a peak-reading means, one lof saidfrequency-selective means passing'said electrical signals falling withina predetermined rumbleA frequency range, the other of saidfrequency-,selective means passing said electrical signals fallingwithin a predetermined engine vibration frequency range different thansaid rumble frequency range, said peak-reading means in each ychannelproducing a signal proportional to the peak-value -of the electricalsignals passed Vby said associated .frequency-selective means,difference measuring means -coupled to both channels for yproducing adifferencefsignal proportional to the difference ybetween the peak-valuesignal `derived ,from the peak-reading means of the respective channelsandutilization means for utilizing said-difference signal which isindicative of the amount-of said characteristic vibration produced bysaid engine.

2. Apparatus according to claim 1, wherein each of saidfrequency-selective mea-ns comprises a pair of AC amplifier stages eachhaving a frequency lrejection network in a negative feedback path, saidamplifier stages and frequency rejection networks being tuned toestablish a predetermined frequency pass-band. v

3. Apparatus according to claim 1, wherein said utilization devicecomprises a peak-.reading meter and hold cir- .cuit for indicating thepeak-value of the difference signal obtained from the differencemeasuring means and holding said value apredeterr'nined time.

4. Apparatus according to claim 1, wherein said peakreading meanscomprises a 'diode lbiased in the forward direction and an RC low-passfilter network connected to said diode to produce a DCsignalproportional to the peak amplitude of the signals passed -by saidrespective frequency-selective means. l y

5. Apparatus according to claim 1, wherein the selected enginecharacteristic is engine rumble, whereinl said frequency-selective.means in said one channel passes electrical signals in saidpredetermined frequency range extending substantially between 850` and,125() cycles per second and said frequency-selective meansin said otherchannel passes electrical signals in said predetermined engine vibrationfrequency range extending substantially between 400 and 700` cycles persecond. y

`6. Apparatus according to claim 1, further comprising means foradjusting the gain in at least one of said channels so that the signals-passed thereby due to engine vibration exclusive of rumble can beadjusted to substantially cancel in the difference measuring means.

7. Apparatus for measuring the rumble in engines according to claim 1,wherein said electrical signals derived from said transducer means areconnected to an amplifier means having a predetermined band-passcharacteristic, a fixed frequency oscillator for generating fixedfrequency signals and modulation means connected to said amplifier meanssuch that the electric signals passed by said amplifier means modulatesaid fixed frequency signal wave in said modulator to produce amodulated outputwave.

8. Apparatus accordingy to claim 7, wherein the selected enginecharacteristic is engine rumble, wherein, the frequencyof said fixedfrequency oscillator is substantially 4000 cycles per second and thefrequency band predetermined by said frequency selecti-ve means in saidone channel extends substantially between 4850 and 5250 cycles persecond, and the frequency band predetermined-by said frequency selectivemeans in said other channelV extends substantially between 4400 and 4700cycles per second.

l9. Apparatus yfor measuring rumble in engines-comprising transducermeans for transforming vibrations of the engine into correspondingelectrical signals, a pair of parallel channels connected to saidtransducer means to receive the electrical signals, a band-passamplifier and a peak-reading circuit connected in each of said channels,said band-'pass amplifier in each channel being set to amplify and passonly signal frequencies lying within different predetermined frequencyranges, the one predetermined frequency range including most of therumble frequencies as well as the engine vibration frequencies fallingwithin the rumble frequency range, the other band-pass amplifierfrequency range including engine vibration frequencies different thanthe rumble frequencies, the peak-reading circuit in each channelproviding an analog signal proportional to the peak amplitude of theelectrical signals passed by the band-pass amplifier in its respectivechan-nel, a differential amplifier connected to =both of said channelsto receive said analog signals and produce a signal output which isproportional to the difference between the received analog signals, saidengine vibration frequencies exclusive of the rumble frequenciescancelling in said differential amplifier leaving the output of saiddifferential amplifier proportional to the rumble, and a peak-readingmeter and a -hold and reset circuit connected to said differentialamplifier for measuring the peak of said rumble output.

10. Apparatus for measuring knock in engines comprising transducer meansfor tarnsforming vibrations of the engine into corresponding electricalsignals, a pair of parallel channels connected to said transducer meansto receive the electrical signals, a band-pass amplifier and apeak-reading circuit connected in each of said channels, .said baud-passamplifier in each channel being set to amplify and pass only signalfrequencies lying within different predetermined frequency ranges, theone predetermined frequency range including most of the knockfrequencies as well as the engine vibration frequencies falling withinthe knock frequency range, the other bandpass amplifier frequency rangeincluding engine vibration frequencies different than the knockfrequencies, lthe peakreading circuit in each channel providing ananalog signal proportional to the peak amplitude of the electrical.signals passed by the band-pass amplifier in its respective channel, adifferential amplifier connected to both of said channels to receivesaid analog signals and produce a signal output which is proportional tothe difference between the received analog signals, said enginevibration frequencies exclusive of the knock frequencies cancelling insaid differential amplifier leaving the output of said differentialamplifier proportional to the knock, and a peak-reading meter and a.hold and reset circuit connected to said differential amplifier formeasuring the peak of said knock output.

11. Apparatus according to claim 10, wherein said frequency-selectivemeans in said one channel passes electrical signals in saidpredetermined knock frequency range extending substantially between 4600and 5600 cycles per second and said frequency-selective means in saidother channel passes electrical signals in said predetermined engineVibration frequency range extending substantially between 2700 and 3700cycles per second.

12. Apparatus according to claim 10, wherein said frequency-selectivemeans in said one channel passes electrical signals in saidpredetermined knock frequency range extending substantially between 1600and 2700 cycles per second and said frequency-selective means in saidother channel passes electrical signals in said predetermined enginevibration frequency range extending substantially between 2700 and 3700cycles per second.

13. Apparatus for measuring rumble and knock in engines comprisingtransducer means for transforming vibrations of the engine intocorresponding electrical signals, three parallel channels connected tosaid transducer means to receive the electrical signals, a band-passamplifier and a peak-reading circuit connected in each of said channels,said band-pass amplifier in each channel being set to amplify and passonly signal frequencies lying within different predetermined frequencyranges, the first predetermined frequency range in the first channelincluding most of the rumble frequencies as well as the engine vibrationfrequencies falling within the rumble frequency range, the secondpredetermined frequency range in the second channel including most ofthe knock frequencies as well as the engine vibration frequenciesfalling within the knock frequency range, and the third band-passamplifier frequency range in the third channel including enginevibration frequencies different than the rumble and knock frequencies,the peak-reading circuit in each channel providing an analog signalproportional to the peak amplitude of the electrical signals passed bythe band-pass amplifier in its respective channel, a first differentialamplifier connected to the output of said first and third channels toreceive said analog signals therefrom and produce a signal which isproportional to the difference between the received analog signals, asecond differential amplifier connected to the output of said second andthird channels to receive said analog signals therefrom and produce asignal output which is proportional to the difference between thereceived analog signals, said engine vibration frequencies exclusive ofthe rumble frequencies cancelling in said first differential amplifierleaving the output of said first differential amplifier proportional tothe rumble, said engine vibration frequencies exclusive of the knockfrequencies cancelling in said second differential amplifier leaving theoutput of said second differential amplifier proportional to the knock,a first hold and reset circuit connected to said first differentialamplifier, a second hold and reset circuit connected to said seconddifferential amplier, a switch means for selecting either said first orsecond hold and reset circuit and a peak-reading meter for measuring thepeak of said rumble or knock output.

References Cited UNITED STATES PATENTS 2,679,746 6/1954 DeBoisblanc73-35 3,024,312 3/1962 Daguet 332-40 X 3,183,708 5/1965 Roddick 73-35RICHARD C. QUEISSER, Primary Examiner.

l. W. MYRACLE, A ssstant Examiner.

